So, I was thinking about the rules as I often do. I thought up a hypothetical that hasn't come up before because none of the people in my play group have try doing the maneuver fighter before.
The situation is that an ally is standing in front of a dragon who has its fire breath charged. I'm on the other side of the dragon not drawing it's attention. My turn comes up right before the dragon and I say "I'd like to hold my action until the dragon aims it's dragon breath. The action I would like to ready is to attack the dragon." Then it's the dragon's turn and the gm states that he will use the dragon's breath attack on my friend, and aims the cone directly towards them. I take my attack and I expend a superiority dice to use maneuvering attack, assuming it hits, and I give my friend half his movement that he can use for his reaction.
Would the dragon be able to re-aim, or would this work out perfectly and render the dragon's breath potentially useless?
The DM can allow this because it is a dramatic move. But one prone to abuse. Because as long as you have superiority dice and acts before the enemy, you could just move an ally.
The RAW order of actions is:
In your turn, you ready an action to attack the dragon when it uses breath weapon.
In the dragon's turn, it breathes fire. Your friend is toasted.
You atack the dragon and the friend (if it is still alive) moves.
This happens in this order because:
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger
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So you cannot interrupt the dragon's breath. You take your readied action after the trigger happens. Your friend will be roasted.
Yes, unfortunately for your friend. I would be fine with letting you "interrupt" the dragon's breath by making your maneuver triggered by the dragon's aiming. However, in 5e, the acting creature has a lot of flexibility on his action, to retarget, to move and swing and maybe move again, depending on the results of his swing, etc. So I would still rule that even though you were able to move your ally, the dragon would still get to breath his weapon at him, unless the movement of your ally put him out of range, of behind full cover, or some such. Even then, if the dragon still had movement left, he could execute more of his move and then still re-aim.